The present invention relates to a method for producing an amorphous alloy ribbon having excellent surface conditions and shape in edge portions, and a method for producing a nano-crystalline alloy ribbon using such an amorphous alloy ribbon.
Liquid-quenching methods are widely known as methods for producing amorphous alloy ribbons for use in magnetic cores, magnetic shields, etc. The liquid-quenching methods include a single roll method, a double roll method, a centrifugal method, etc., and preferable among them from the aspect of productivity and the maintenance of an apparatus is a single roll method in which a molten metal is supplied onto a cooling roll rotating at a high speed and rapidly quenched to form an alloy ribbon.
FIG. 1 shows one example of apparatuses for carrying out the single roll method. An alloy ingot in a crucible 1 is melted by a high-frequency coil 2, and the resultant alloy melt 3 is ejected through a nozzle 4 onto a cooling roll 5 rotating at a high speed and rapidly quenched to form an amorphous alloy ribbon 6. As shown in FIG. 1, for instance, a high-pressure gas such as nitrogen, a compressed air, etc. is supplied from a peeling-gas nozzle 7 in an opposite direction to the rotation direction of the cooling roll 5 immediately after the casting, thereby forcedly peeling the amorphous alloy ribbon 6 from the cooling roll 5.
The amorphous alloy ribbon 6 produced by the above method tends to be provided with small dents called xe2x80x9cair pocketsxe2x80x9d on a side in contact with the cooling roll 5. This is because a gas is entrained into a boundary between a melt pool portion 10 (hereinafter referred to as xe2x80x9cpaddlexe2x80x9d) and the cooling roll 5 by the rotation of the cooling roll 5, so that it expands in the paddle 10 until the melt is solidified. Because the formation of such air pockets leads to the roughing of a surface of the ribbon 6, the air pockets should be as few as possible.
Proposed by German Patent DD266046A1, Japanese Patent Laid-Open No. 6-292950, etc. to suppress the formation of air pockets is a method in which a CO2 gas is supplied to the paddle in various directions. This method is advantageous in that it can suppress the formation of air pockets to reduce the surface roughness of a ribbon on a side in contact with the cooling roll.
Alternatively, Japanese Patent Laid-Open No. 59-209457, Japanese Patent Publication No. 1-501924, etc. propose a method for producing an amorphous alloy ribbon in vacuum or in a He atmosphere, a method for producing an amorphous alloy ribbon while flowing a gas having a lower density than the air at normal temperature, such as a heated CO gas, a He gas at normal temperature etc., to the paddle from rearward. Why the formation of air pockets can be suppressed by these methods seems to be the fact that a gas entrained by the rotation of the cooling roll has a reduced density, resulting in decrease in the kinetic pressure of the gas impinging on the paddle, thereby suppressing the vibration of the paddle.
Among the above methods, the method of flowing a CO2 gas to the paddle from rearward (from a side opposite to the side on which the ribbon is formed) is suitable for the mass production of amorphous alloy ribbons from the aspect of production cost and safety.
The total length of an amorphous alloy ribbon continuously produced in one casting lot by a liquid-quenching method generally exceeds 3,000 m. When the resultant amorphous alloy ribbon is wound around a reel after the completion of casting, the ribbon is likely to be twisted. Accordingly, the quenched ribbon should continuously be wound immediately after peeling from the cooling roll.
For instance, Japanese Patent Laid-Open Nos. 8-318352 and 11-188458 disclose a method in which a magnetized reel rotating in an opposite direction to a cooling roll is positioned near the cooling roll to magnetically attract the peeled ribbon, which is continuously wound around the reel.
It is also known that the heat treatment of an amorphous alloy ribbon produced by the above-described methods at a temperature equal to or higher than the crystallization temperature of the alloy can provide a nano-crystalline alloy ribbon having an average particle size of 100 nm or less. Typical alloys capable of forming nano-crystalline alloy ribbons are Fexe2x80x94Sixe2x80x94Bxe2x80x94(Nb, Ti, Hf, Mo, W, Ta)xe2x80x94Cu alloys, Fexe2x80x94(Co, Ni)xe2x80x94Cuxe2x80x94Sixe2x80x94Bxe2x80x94(Nb, W, Ta, Zr, Hf, Ti, Mo) alloys, Fexe2x80x94(Hf, Nb, Zr)xe2x80x94B alloys, Fexe2x80x94Cuxe2x80x94(Hf, Nb, Zr)xe2x80x94B alloys, etc. described in Japanese Patent Publication Nos. 4-4393 and 7-74419, Japanese Patent 2,812,574, etc.
The nano-crystalline alloys are not only substantially free from thermal instability unlike the amorphous alloy, but also are subjected to less change with time and have lower magnetostriction and higher permeability than the amorphous alloys, they are used for common-mode choke coils, pulse transformers, leakage breakers, etc.
As a result of experiment to produce an amorphous alloy ribbon 6 using a laboratory-scale apparatus whose casting time is less than 30 seconds while flowing a CO2 gas, the resultant ribbon had excellent surface conditions. However, in a production experiment using a mass-production-scale apparatus, it was found that as the casting time passed, there arose the problems of embrittlement and crystallization in the formed amorphous alloy ribbon that were not observed in the short casting process, though the surface conditions of the amorphous alloy ribbon was improved by the supply of a CO2 gas. In addition to these problems, it has also be found that a new problem of serrated irregular shapes in their edge portions takes place. This phenomenon never occurs even in the long casting process of an amorphous alloy ribbon unless a CO2 gas is supplied.
Because the total length of an amorphous alloy ribbon continuously produced in one casting step by a mass-production apparatus exceeds 3,000 m, the amorphous alloy ribbon is continuously wound around a large reel during the casting from the aspect of efficiency. The ribbon is then divided to proper length that can easily be handled to produce wound cores, etc., and wound around a large number of small reels. At this time, if the ribbon had a serrated irregular shape in its edge portions, the edge portion of the ribbon engages a reel, resulting in extreme difficulty in handling.
The irregular shape of the ribbon in its edge portions also poses inconveniences in the production of a wound core. In the continuous production of a wound core from a ribbon, the winding of the ribbon is often carried out with the edge portions of the ribbon abutting against a plate to make the resultant wound core have a constant height. In this case, too, if the ribbon had a serrated irregular shape in its edge portions, the ribbon engages the abutting plate, thereby making the production of a wound core difficult.
If the ribbon were brittle, breakage, cracking, etc. would be likely to occur in the production of wound cores and laminated cores. In addition, if the ribbon contains coarse crystals, it would have large crystal magnetic anisotropy, resulting in the deterioration of its soft magnetic properties. Further, if an amorphous alloy ribbon having coarse crystals were heat-treated at a temperature equal to or higher than the crystallization temperature of the alloy, the resultant nano-crystalline alloy ribbon would have deteriorated soft magnetic properties.
Accordingly, an object of the present invention is to provide a method for continuously producing an amorphous alloy ribbon having improved surface conditions on a side in contact with a cooling roll and excellent edge shapes, free from embrittlement and crystallization.
Another object of the present invention is to provide a method for producing a nano-crystalline alloy ribbon by heat-treating such an amorphous alloy ribbon.
As a result of investigating problems such as irregular edge shapes, embrittlement and crystallization occurring as the amount of a ribbon cast increases (as casting continues longer) in the production of an amorphous alloy ribbon with a CO2 gas supplied, the inventors have found that the above problems can be overcome by grinding a cooling roll during the casting. The present invention has been completed based on this finding.
Thus, the method for producing an amorphous alloy ribbon by ejecting an alloy melt onto a cooling roll and rapidly quenching it according to the present invention comprises carrying out the grinding of the cooling roll while supplying a gas based on CO2 near a paddle of said alloy melt during the casting.
The alloy used in the present invention preferably has a composition comprising 13 atomic % or less of B and 15 atomic % or less of at least one selected from the group consisting of transition elements of Groups 4A, 5A and 6A, the balance being substantially Fe. Also, when the amorphous alloy ribbon is to be heat-treated for nano-crystallization, the alloy melt preferably contains 3 atomic % or less of at least one of Cu, Ag and Au.
When a gas based on CO2 is supplied near a paddle of an alloy melt ejected from a nozzle onto a cooling roll immediately after the start of casting, the ribbon tends to be broken. However, when the gas based on CO2 starts to be supplied after the surface temperature of the cooling roll has become substantially constant, the possibility of the breakage of the ribbon substantially decreases. Here, xe2x80x9csurface temperature has become substantially constantxe2x80x9d means that the variation range of the surface temperature of the cooling roll has become within 10xc2x0 C. relative to its average temperature. Though the surface temperature of the cooling roll generally starts to elevate immediately after the start of casting, it becomes substantially constant in several seconds to ten and several seconds because heat from the alloy melt gets balanced with heat dissipating from the cooling roll.
The peripheral speed of the cooling roll is preferably 35 m/second or less, more preferably 20-30 m/second. The temperature of the alloy melt is preferably from the melting point of the alloy +50xc2x0 C. to the melting point of the alloy +250xc2x0 C., more preferably from the melting point of the alloy +100xc2x0 C. to the melting point of the alloy +200xc2x0 C. In addition, the distance from a tip end of a nozzle to a cooling roll is preferably 200 xcexcm or less, more preferably 100-180 xcexcm, further preferably 100-150 xcexcm. Under such casting conditions, it is possible to stably produce an amorphous alloy ribbon having a thickness of 8-25 xcexcm, particularly 8-19 xcexcm.
The preferred method of the present invention for producing an amorphous alloy ribbon by ejecting an alloy melt onto a cooling roll and rapidly quenching it comprises (a) preparing an alloy melt having a composition comprising 13 atomic % or less of B and 15 atomic % or less of at least one selected from the group consisting of transition elements of Groups 4A, 5A and 6A, the balance being substantially Fe; (b) ejecting the alloy melt at a temperature from the melting point of the alloy +50xc2x0 C. to the melting point of the alloy +250xc2x0 C. through a nozzle onto the cooling roll rotating at a peripheral speed of 35 m/second or less, a distance between a tip end of the nozzle and the cooling roll being 200 xcexcm or less; (c) starting to supply a gas based on CO2 to the alloy melt after the surface temperature of the cooling roll has become substantially constant; and (d) grinding the cooling roll while supplying the gas based on CO2.
The heat treatment of the amorphous alloy ribbon produced by the above method at a temperature equal to or higher than the crystallization temperature of the alloy can provide a nano-crystalline alloy ribbon having a nano-crystalline structure having an average particle size of 100 nm or less.